Development of a partially nonergodic ground-motion model for Fourier amplitude spectra for the San Francisco Bay Area, California

Morgan P. Moschetti, Eric M. Thompson, Ryan Peterson, Davis Engler, James A. Smith, Brad T. Aagaard, & Oliver S. Boyd

Published September 10, 2023, SCEC Contribution #13225, 2023 SCEC Annual Meeting Poster #189

We develop a partially nonergodic ground-motion model (GMM) for Fourier amplitude spectra for the San Francisco Bay Area, California using Bayless and Abrahamson (2019) as a reference ergodic GMM and developing location-dependent adjustments to the median prediction and to its variance. We compile regional ground-motion data from M>3 (all magnitude types) earthquakes occurring 2000–2022 for which moment magnitudes are available in the U.S. Geological Survey Comprehensive Catalog and retain events with measured moment magnitudes. The data set predominantly consists of records from M_W3.5–4.5 earthquakes but includes three well-recorded M_W>5 events. Ground-motion residuals are evaluated using V_S30 values from the California-specific model of Thompson et al. (2022) and basin-depth site parameters from the seismic velocity model of Aagaard and Hirakawa (2021). The V_S30 and basin-depth scaling model of the reference ergodic GMM are evaluated and modified with the updated data set. We compute maps of site adjustments using a varying-coefficient model that considers the spatial correlation structure of the observations and the uncertainties at each location. The varying-coefficient model is developed from maximum likelihood fits to the means and standard deviations of the standardized site residuals. Due to the predominance of small- to moderate-magnitude earthquakes in the data set, we do not modify between-event variance. Amplification effects correlate with known geologic structure in the region and highlight deviations from the empirical GMM for the region. This approach allows for incorporation of models of within-event variability that depend on magnitude, distance, or V_S30. Conditional predictions of the mean and standard deviation use the empirical within-event variability of the GMM. This approach ensures that the predictions at locations away from observations asymptotically approach those of the GMM. These results highlight important regional amplification effects and could ultimately be used in seismic hazards assessments.

Key Words
San Francisco Bay Area, nonergodic, Fourier

Citation
Moschetti, M. P., Thompson, E. M., Peterson, R., Engler, D., Smith, J. A., Aagaard, B. T., & Boyd, O. S. (2023, 09). Development of a partially nonergodic ground-motion model for Fourier amplitude spectra for the San Francisco Bay Area, California . Poster Presentation at 2023 SCEC Annual Meeting.

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